Salmonellosis continues to be a major infectious disease in both the United States and elsewhere. The overall goal of this project is to gain a better understanding of the early events that occur during Salmonella infections. The proposed studies focus on the initial interaction of salmonellae with human-derived cells: intestinal epithelial cells and macrophages. The first objective is to determine the role of the porins OmpC and OmpD in mediating attachment of S. typhimurium to and internalization by these host cells. In these studies mutants deficient in one or both porin genes will be compared to the parental wild type strain in their ability to adhere to and become internalized by target host cells. Bacteria-eukaryotic association will be measured by fluorescence microscopy and flow cytometry. The second objective is to determine the ability of porins or porin-lipopolysaccharide complexes to inhibit bacterial attachment to host cells. Partially purified porins, containing associated lipopolysaccharide, and highly purified porins will be compared for the respective ability to block attachment, again using fluorescence microscopy and flow cytometry. The third objective will use non-targeted approaches to identify major bacterial ligands involved in interactions with target human cells. Potential molecules will be identified by probing bacterial envelope proteins with labeled host cells and by recovering Salmonella-derived binding protein by elution from host cells. Antiserum generated against these proteins will be used in binding inhibition studies. These studies are expected to provide a better understanding of the early cellular events in Salmonella infections. Such information may prove useful in developing microbial vaccines both for homologous protection and for hybrid immunogens that use Salmonella as a vector.